The present invention is directed to bicycle transmissions and, more particularly, to bicycle transmissions internally mounted within a wheel hub.
A typical internally mounted bicycle transmission is disclosed in Japanese Utility Model Application 58-10711. The hub disclosed in that document includes a hub axle, a driver rotatably supported by the hub axle, and ring and planet gears serving as transmission paths for transmitting power to the hub body. A transmission pawl which may assume either an upright state or a reclined state is used to couple the rotational power from the driver to the hub body. More specifically, the transmission pawl transmits the rotational power of the driver to the hub body when the transmission pawl is in an upright state, typically through engagement of the transmission pawl with the inner periphery of the hub body, whereas the rotational power of the driver is not transmitted to the hub body when the transmission pawl is in the retracted state. An operating element for putting the transmission pawl into the upright state or the reclined state is operated by the displacement of a shifting rod in the direction of the longitudinal axis of the hub axle. In conventional hub transmissions, once the operating element is set to cause the transmission pawl to be in the upright state, the transmission pawl is maintained in the upright state regardless of whether driving power is being applied by the driver.
Most internal hubs are provided with coaster brakes, wherein the brakes are applied by reversing the rotation of the pedals. More specifically, when the rotation of the pedals is reversed and that of the driver is reversed to operate the coaster brakes, the brakes are applied to prevent rotation of the hub body. The pedals must be rotated in the forward direction to release the brakes and apply drive power to the rear wheel. However, if the transmission pawl is in an upright state, the transmission pawl may engage the hub body while the brakes are applied. Since the brakes prevent forward rotation of the hub body, and since the pedals are coupled to the hub body through the transmission pawl, the pedals cannot be rotated forward to release the brakes. This results in the problem referred to as brake lock, where the brakes cannot be released.
Another problem is caused by the constant engagement of the pawl with the hub body when the bicycle rolls back on an incline such as a hill. In this case the rotation of the wheel is transmitted by the pawl to the planetary gear mechanism and prevents the coaster brake from operating unless the cyclist can rotate the pedals backwardly at a faster rate than the wheels. In reality the coaster brake is almost impossible to operate in this situation.
Even in bicycles which are not equipped with coaster brakes, when the transmission pawl is brought into contact with the hub body during the reverse rotation of the driver, the transmission pawl comes into contact with the hub body, resulting in the problem of noise.